The present invention relates to fine-grained cemented carbides with a binder phase enriched surface zone, a so-called gradient zone. The gradient zone is essentially free from cubic carbides or carbonitrides that can form due to the addition of grain growth inhibitors. Yet, the gradient zone is fine grained.
Coated cemented carbide inserts with binder phase enriched surface zone are today used to a great extent for machining of steel and stainless materials. Thanks to the binder phase enriched surface zone, an extension of the application area for cutting tool material has been obtained.
Methods or processes to make a cemented carbide containing WC, cubic phase (carbonitride) and binder phase with binder phase enriched surface zones are within the techniques referred to as gradient sintering and are known through a number of patents and patent applications. According to U.S. Pat. Nos. 4,277,283 and 4,610,931 nitrogen containing additions are used and sintering takes place in vacuum whereas according to U.S. Pat. No. 4,548,786 the nitrogen is added as a gas. In both cases a binder phase enriched surface zone essentially depleted of cubic phase is obtained. U.S. Pat. No. 4,830,930 describes a binder phase enrichment obtained through decarburization after the sintering whereby binder phase enrichment is obtained which also contains cubic phase.
In U.S. Pat. No. 4,649,084, nitrogen gas is used in connection with sintering in order to eliminate a process step and to improve the adhesion of a subsequently deposited oxide coating. In patent EP-A-0569696 the binder phase enriched zone is obtained with the presence of Hf and/or Zr. In patent EP-0737756 the same effect is achieved with Ti present in the cemented carbide. In these patents, it is shown that cubic carbide formers of group 4A (Ti, Zr, Hf) can be used to achieve a binder phase enriched surface zone.
From a fracture mechanical point of view, an enrichment of binder metal in a surface zone means that the ability of the cemented carbide to absorb deformation and stop growing cracks from propagating. In this way a material is obtained with improved ability to resist fracture by allowing greater deformations or by preventing cracks from growing, compared to a material with mainly the same composition but homogenous structure. The cutting material, thus, exhibits a tougher behavior.
Cemented carbide inserts with a submicron structure are today used to a great extent for machining of steel, stainless steels and heat resistant alloys in applications with high demands on both toughness and wear resistance. In order to maintain the grain size during sintering such cemented carbide generally contains grain growth inhibitors. Common grain growth inhibitors include vanadium, chromium, tantalum, niobium and/or titanium or compounds involving these. The strongest inhibition is obtained using vanadium and/or chromium. When added, generally as carbides, they limit grain growth during sintering, but they also have undesirable side effects. Precipitation of unwanted brittle structure components affects the toughness behaviour in an unfavourable direction.